In the normal art, Wheatstone or Wheatstone-derived bridges are constructed with a single output signal generated by the bridge. This signal is used either for measurement or control. Usually one of the four arms becomes a sensor or detector, although occasionally two opposite arms employ separate sensors for increased sensitivity. If it should be desired to provide two output signals at proximate but different levels of the signal, say to define a control or measurement band in a parameter, then two different bridges are used, which requires a separate sensor for each bridge. Rarely, a bridge will contain six arms (of which two or more are sensing) arranged with two of the arms on one side of the typical bridge diamond and four on the other side, but the four elements interplay for self-compensation purposes and only one signal emanates from the bridge. Quite often a four-arm bridge will have five elements in it, the fifth element being a variable resistor between two arms, but the fifth element is then center-tapped, such that the bridge has only one output signal (e.g., the Kelvin double-bridge).